Electronic fingerprint sensors are used on consumer products to perform many different tasks. They are used to authenticate and verify users; to emulate other input devices such as computer mice, joy sticks, scroll bars, and pressure-sensitive push buttons; and to launch software programs, each correlated to a specific fingerprint. A fingerprint sensor on a product can be used many times during a day, to perform any one or more of these tasks. Thus, the contact surface of a fingerprint sensor is exposed to constant wear. This is especially true for finger swipe sensors, which are subjected to constant tapping and swiping.
Some finger swipe sensors employ a top metal layer. This top metal layer can perform many functions such as providing electrostatic discharge (ESD) protection or acting as an RF antennae, to name a few uses. FIG. 1 is a side cross-sectional view of a finger swipe sensor 100 in the prior art, which employs a top metal layer. The finger swipe sensor 100 has a layer 110 containing one or more contact sensing elements 125. The layer 110 has a top surface 110A positioned under a metal layer 115. Typically, a finger 105 is swiped across a top surface of the metal layer 115, applying forces to the metal layer 115. Such externally applied forces induce plastic deformation in the metal layer 115, as illustrated by the lines of dislocations 120. Plastic deformation occurs through the creation and movement of many different types of crystallographic defects. For simplicity of the discussion and because most crystallographic defects can be modeled by dislocations, dislocation creation and movement are used as the primary mechanism through which plastic deformation occurs.
FIG. 2 is a side cross-sectional view of the finger swipe sensor 100, after repeated swiping across a surface of the metal layer 115. As illustrated in FIG. 2, dislocations 120 that can move collect at the right end of the metal layer 115 thus plastically (irreversibly) deforming the metal layer 115. Dislocations produced by finger swiping can move through the metal layer 115 but are stopped by the metal edges, where they collect and coalesce. Dislocation coalescence produces voids along the metal edges. The metal edges become brittle from the collected dislocations and more prone to fracture along the line 117. The fractured metal is very likely permanently removed from the sensor 100, thus contributing to the observed abrasion and erosion effect of repeated finger swiping. Furthermore, any erosion on the metal layer 115 will continue, so that the rightmost edge of the metal layer 115 will creep in the direction shown by the arrow labeled Y.